The present invention generally relates to a supercharged automobile engine and, more particularly, to an air intake system for the supercharged automobile engine of a type having primary and secondary throttle valves downstream of the supercharger.
Hitherto, an air intake system for an internal combustion engine comprising primary and secondary throttle valves disposed in air intake passage means has been known. Such a prior art air intake system is so designed that, while the opening of the primary throttle valve is controlled by an accelerator pedal to which it is operatively coupled, the secondary throttle valve has its opening controlled in dependence on the amount of air being sucked into the engine cylinders to effect a multi-stage control of the amount of the air, thereby improving the metering precision for the amount of the air, the workability of the accelerator pedal during low speed drive, and the negative pressure characteristic of the vacuum hall in the exhaust gas recirculating system and/or the distributor.
However, when it comes to a supercharged internal combustion engine, there is such a limitation that, in order to avoid the possibility that a lubricant oil used to lubricate the supercharger may leak into the air intake passage means under the influence of the negative pressure developed therein during low load operating condition of the engine, the primary and secondary throttle valves must be located downstream of the supercharger with respect to the direction of flow of air or an air-fuel mixture towards the engine cylinder.
In view of the above, in the well known system wherein the secondary throttle is so designed as to be controlled in dependence on the negative pressure developed across the venturi area in the fuel intake passage means upstream of the primary throttle valve. The negative pressure across the venturi area tends to be counterbalanced by the pressure of supercharged air into the fuel intake passage means downstream of the venturi area and, therefore, the secondary throttle valve can hardly be controlled to a proper opening.
Because of the reason which has been stated above, the prior art supercharged automobile engine actually embodied in currently commercially available automobiles employs a linkage type in the air intake system, which linkage type is so designed that, while the primary throttle valve is operatively coupled to the accelerator pedal, the secondary throttle valve is linked to the primary throttle valve so that, when and after the primary throttle valve has been adjusted to a predetermined opening, for example, pivoted about 40.degree. from its substantially closed position towards the full open position, the secondary throttle valve can be forcibly opened. However, this type has been found disadvantageous by the reason which will now be described.
As stated above, in the linkage type, the secondary throttle valve is opened irrespective of the amount of the air being sucked, when and after the primary throttle valve has been adjusted to an opening equal to or greater than the predetermined value. Therefore, during acceleration occurring at the time of start of the automobile, both of the primary and secondary throttle valves are immediately adjusted to the full open position, accompanied by an abrupt increase of the amount of air being sucked. Once this abrupt increase of the amount of the air takes place, the air flow sensor used to detect the flow rate of air in wide range tends to fail temporarily to respond with consequent reduction in follow-up characteristic or consistency of the fuel supply. This in turn results in that the air-fuel mixture being supplied into the engine is unnecessarily leaned to such an extent as to bring about the faltered acceleration and also as to bring about reduction in response to the acceleration, i.e., reduction in acceleration characteristic.
In an attempt to substantially eliminate the above discussed problems, the Japanese Utility Model Application No. 55-79051, filed on June 5, 1980 in the name of the assignee of the present invention and subsequently laid open to public inspection on Jan. 7, 1982 under Laid-open Utility Model Publication No. 57-2225, discloses a similar fuel intake system for a supercharged automobile engine which employs a pressure responsive control such as a diaphragm valve assembly so designed and so positioned as to detect the pressure dominant in the fuel intake passage means downstream of the throttle valve and as to increase the opening of the secondary throttle valve in response to the increase of the detected pressure. While this prior system appears to be satisfactory, a different disadvantage has been found in that, since the positive pressure introduced into one of the working chambers of the diaphragm valve assembly forming the pressure responsive control fails to readily evacuate therefrom during deceleration of the engine and, also, since the pressure of the supercharged air discharged into the fuel intake passage means from the superchargee does not readily fall also during deceleration of the engine because of inertia action of the supercharger, the response of the secondary throttle valve to return towards the closed position during the deceleration of the engine, that is, the response to deceleration, is apt to be adversely affected.
Apart from the above, the Japanese Patent Publication No. 50-12048, published on May 8, 1975, discloses an air intake system specifically so designed as to substantially eliminate the occurrence of the faltered acceleration of the automobile engine. According to this publication, the air intake system comprises primary and secondary intake passages, the primary intake passage having a primary throttle valve disposed therein and adapted to be controlled by the accelerator pedal, whereas the secondary intake passage has a secondary throttle valve and an auxiliary throttle valve both disposed therein with the auxiliary throttle valve positioned upstream of the secondary throttle valve. While the secondary throttle valve is so operatively coupled to the primary throttle valve that, only when and after the primary throttle valve has been opened to a predetermined opening, the secondary throttle valve is opened, the auxiliary throttle valve is eccentrically supported with its pivot axle extending off the center of the auxiliary throttle valve and is normally held in position to close the secondary intake passage by the action of a weight secured to one end of the pivot axle exteriorly of the secondary intake passage.
The auxiliary throttle valve used in the prior art air intake system disclosed in the above mentioned Japanese patent publication is of a type responsive to the negative pressure developed inside the secondary intake passage such that, a predeterminal time after the opening of the secondary throttle valve, that is, when and after the negative pressure developed downstream of the auxiliary throttle valve and upstream of the secondary throttle valve subsequent to the opening of the secondary throttle valve, has attained a magnitude sufficient to overcome the weight acting on the auxiliary throttle valve, the latter is brought in position to open the secondary intake passage. Therefore, the air to be supplied towards the engine cylinder would not be abruptly and prematurely increased substantially simultaneously with the opening of the secondary throttle valve, but the predetermined time after the opening thereof, thereby substantially eliminating the occurrence of the faltered acceleration of the engine which would tend to take place during the acceleration at the time of start of the automobile.
In addition to the substantial elimination of the possible occurrence of the faltered acceleration, this prior art air intake system has an additional advantage in that, since the secondary throttle valve is closed in response to the closure of the primary throttle valve, during the deceleration, a good response to the deceleration can be appreciated.
However, this Japanese patent publication is silent as to the applicability of the air intake system disclosed therein to the supercharged automobile engine. In addition, since this prior art air intake system employs the weight necessary to normally close the auxiliary throttle valve having pressure receiving regions of different surface areas on respective sides of the pivot axle, the auxiliary throttle valve may provide a resistance to the flow of the air to such an extent as to result in reduction of the engine power output in a magnitude corresponding to such resistance, thereby posing a problem.
In addition to the above described problem, the prior art air intake system has an additional disadvantage in that, since both of the secondary and auxiliary throttle valves tend to be readily closed at the time of temporary reduction in load such as occurring during the release of the driver's foot from the accelerator pedal for effecting a gear change, the opening of the auxiliary throttle valve takes place gradually even upon pedaling of the accelerator pedal effected after the gear change and, therefore, the engine power output is apt to be temporarily reduced.